In environments that are unsuitable or hostile for humans, robots are particularly suited to perform tasks that would otherwise be performed by humans. An example of such an inhospitable environment that would benefit from robotics is subsurface mapping, such as the mapping of a tunnel or cavity underground.
A specific application of subsurface mapping is the charting and mapping of tunnels such as sewer tunnels. In such subsurface environments, ventilation is poor and the environment may be partially or fully flooded with water and/or liquid waste. As a result, personnel entry is either forbidden, or requires the use of specialized ventilators.
Many regions are faced with trying to accommodate current, as well as old and dis-used, tunnel systems. It would be advantageous to use a robotic system for charting and/or mapping a subsurface cavity. It would also be advantageous to control subsurface robots in the system with a navigation system that employs avionics.
Navigation systems for the operation and geo-positioning of aircraft, watercraft and land-based vehicles are well known in the art. In general, many navigation systems include one or more subsystems that are integrated to provide the most accurate positioning possible. Often, inertial navigation systems are used with one or more sensors to calculate the position, orientation and velocity as a moving object as it travels. However, subsurface navigation presents challenges that are not encountered with traditional surface (land or water) or aerial navigation systems due to the inherent restrictions on line-of-sight communication in the subsurface and the limitations on data transmission through the subsurface environment.
Inertial navigation systems use local measurements from on-board sensors over time and certain algorithms to produce a best estimate of position as an object moves from an initial position. Sensors known in the art as “dead-reckoning” sensors measure acceleration and angular velocity from which integrative functions produce a distance and vector relative to an initial reference coordinate position. A known problem with inertial navigation systems is the deterioration of accuracy over time due to the accumulation of unbounded errors with each measurement.
To compensate for these unbounded errors, many navigation systems for surface or aerial applications augment the inertial system with a complementary system, such as a positioning system, that aids in reducing the effect of accumulating measurement errors. For example, Global Positioning Systems (GPS) are often used in commercial airplane avionics to periodically update and correct the inertial system with external position measurements.
While inertial navigation systems have been used for subsurface applications, traditional GPS has not been used to compensate for the problems associated with unbounded errors in the subsurface data transmissions given that the ability to obtain an accurate geographical position using GPS requires triangulation of signals from at least three satellites by line-of-sight transmission from each satellite to the GPS receiver. As a result, inertial systems have found limited application in the subsurface and are known to have limited accuracy that degrades as the distance from the initial reference point increases spatially and temporally.
It would be advantageous to provide a subsurface avionics system that includes an inertial navigation system or some other means of navigation that can be supplemented with a means of external reference for accurate real-time positional re-calibration of the inertial navigation system.